I think IQ is useful in aggregate (for example, a finding that exposure to local toxins reduces a cities' performance on IQ by 10 points), but not useful an an individual level (e.g. you have an IQ of 130, so we can say with certainty you will earn $30,000 more per year). It's similar with MRI scans of ADHD: they find brain differences at a large scale, but you can't use a MRI to diagnose ADHD.